About 10 years ago I discovered epoxy primer. It was recommended for rust repair, and later as a primer for restoration. Epoxy primer seals metal, preventing further oxidation of the bare metal. The problem with conventional primer is that it does not protect from moisture, it is porous and literally absorbs moisture like a sponge.

For rust repair, conventional primer does not provide adequate protection against the further advancement of corrosion. For restoration and body work, epoxy primer is an excellent way to coat and protect bare steel. If while block sanding I sand through the primer back to bare metal, I will apply another coat of epoxy primer over the bare metal to ensure there is a coat of epoxy primer protecting all the bare metal.

I have rust repair work almost ten years old that is still holding up quite well, holding up better than the factory corrosion protection and paint.

For years I have been coating metal with epoxy primer after I have done any hammering and welding. Clean, bare metal is epoxy primed, then all my body work is done OVER the epoxy primer. The primer instruction sheet will provide instructions describing the adhesion window for body filler or subsequent paint of the primer. There is usually a time window of a number of hours before the primer must be sanded and recoated. Filler instructions state that it must be applied to bare metal. I have not had any adhesion problems applying filler over fresh epoxy primer.

Body filler is also porous, and absorbs water. It will cause extensive corrosion if it comes in contact with moisture and is applied over bare metal. This will easily ruin even the best paint job.

Epoxy primer also makes an excellent sealer too. I use reduced epoxy primer as a sealer before applying the color coat. This provides a moisture proof seal against any body work and primer-surfacer, protecting the body work and the metal underneath. Epoxy primer-sealer also prevents those nasty sanding marks from appearing months later. Conventional primer is somewhat plastic, and shrinks and settles for months after being sprayed. Instructions for using the primer as a sealer are included in the instruction sheet.

I have been using PPG paint products for years, and have been impressed with the results. I use PPG DP-40LF Epoxy Primer. It has proven to be quite effective at stopping the further advancement of rust after rust repair, and I have not had any adhesion problems or other failures with this primer.
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